By way of background, snack foods are commonly prepared or cooked in an hot oil bath and upon being removed from the cooking oil bath, a quantity of cooking oil is carried out adhering to the surfaces of the snack food products. In many cases the “carry-out” of the cooking oil is undesirable both from the standpoint of ultimate product taste as well as the cost of cooking oil which may often be more costly than the base snack product.
Conventionally these products have been de-oiled in batch type centrifuges. A batch centrifuge employs a generally cylindrical or conical basket rotatable about its vertical axis. In operation, the basket is filled while in the stationary condition and the product charge is retained by a discharge gate or valve in the bottom of the basket. The basket is then rotated at a speed sufficient to produce a high centrifugal acceleration, typically on the order of between 30 and 60 g or even higher, at its periphery. The high speed rotation may be maintained for as little as 1 or 2 seconds or for 15 seconds or more depending on the nature of the product and the degree of de-oiling required. The rotation of the basket is then stopped and the discharge gate or valve is opened to discharge the batch. Finally, the gate or valve is closed and the basket may be re-filled with the next batch of product to be de-oiled. Typically the batch type centrifuge must be integrated into a continuous process system wherein several disadvantages may be encountered.
One disadvantage is that a batching hopper or other means of surge accumulation must be provided to control the flow of product into the batch centrifuge. Another disadvantage is that frequently de-oiling by centrifuge is time critical being that the product must be spun as soon as practical after frying and before it begins to cool. It is understood that as the product cools the oil may become more viscous and/or be absorbed into the product, thereby inhibiting its removal by centrifuging. Obviously, the age difference between the oldest and newest product in a batch entering the centrifuge must be some time interval greater than the actual de-oiling time, i.e. the time spent under high centrifugal acceleration. Thus we see that the degree of de-oiling may vary within a batch depending on the age of each portion of the batch. Yet another disadvantage is the perception of many equipment customers that a batch process renders non-continuous an otherwise continuous process. Having regard to the variable time element described above, that perception is quite valid.
A centrifuge of the purely continuous type would appear to be a solution to the above disadvantages if it were capable of being successfully integrated into a snack food processing line for products of the type described above. In the prior art there many known types of continuous centrifuges. The milk clarifying centrifuge of U.S. Pat. No. 2,264,665, and those like it, served to separate liquids of different densities. U.S. Pat. Nos. 4,205,999 and 6,267,899 disclose apparatus and processes for separating liquids from solids or to recover liquids while discharging solid contaminants as waste. Also such apparatus may be adapted to the recovery of solids when those solids are sufficiently robust to survive the discharge process.
Prior art continuous centrifuges typically rotate at constant speed. Therefore both liquid and—of particular significance—solid fractions must exit the rotor in a state of high kinetic energy. A number of known centrifuges such as those disclosed in U.S. Pat. Nos. 4,462,570 and 6,521,120 discharge the solid components at a point of maximum rotor internal diameter and in these cases the kinetic energy will be very high. Fried snack food products of the types to be treated by the present invention are quite fragile. Upon discharge from one of the prior art centrifuges they would shatter or be severely damaged through impact with the centrifuge static shroud or outer wall. This issue was recognized in U.S. Pat. No. 6,267,899 wherein particular deflection structure was disclosed to ameliorate the impact forces upon the sugar crystals in the discharge step. A further limitation of such a centrifuge is the very short residence time for the solids in the rotor and the resulting de-oiling would be minimal. U.S. Pat. Nos. 5,160,441 and 6,712751 disclose conveying the solid fraction mechanically so as to discharge closer to the rotor axis wherein the solid factions would exit with reduced kinetic energy. In the interest of achieving a good throughput capacity, the discharge port of a practical rotor must be of a reasonable diameter but nevertheless the solids would still exit with a significant kinetic energy. And additionally, forcing the product to traverse the inner surface of the rotor under the influence of very high gravitational forces would be a source of product damage. We believe the problem of product damage upon discharge is solved through the cyclic rotational speed of a quasi-continuous centrifuge wherein the product is discharged only at very low rotational speeds and with very low kinetic energy.